1. Field of the Invention
The field of art to which this invention pertains is tenter frames for drawing plastic films and, more particularly, it is directed to methods and apparatus for drawing such films using linear motors.
Specifically, this invention involves drawing film by propelling active carriages, with tenter clips attached, with synchronous linear motors. These same motors are also used to propel stacks of these carriages, at controlled speeds, in collection sections prior to entering the tenter frame. Hysteresis linear motors are used to propel the carriages into the stacks, and further provide a means of propelling passive carriages with idler clips attached along the return sides of the tenter frame, after completion of the drawing operation.
2. Description of the Related Art
As just indicated, the method and apparatus of this invention are primarily used to stretch or draw a web of sheet material or film in a tenter frame. The film is drawn in the machine direction (MD) by propelling tenter clips, which grip the film, in pairs along opposed tracks at ever increasing velocities to space the pairs of clips from each other and thereby draw the film longitudinally. Transverse direction (TD) drawing occurs as the clips follow diverging portions of the tracks.
Typical methods for drawing film in this manner are shown in U.S. Pat. No. 3,890,421 to Habozit; in Japanese patent publication 48-38779; and, in the French patent 2,317,076. These patents and publication, however, do not teach the carefully coordinated controls required, in drawing film in accordance with the instant invention. In such invention, pairs of clips which are directly opposite each other, are propelled, while maintaining this opposite positioning, at identical velocities and precise spacings with adjacent, opposed, tenter clip pairs. This operation is accomplished by use of synchronous linear motors.
More specifically, in the tenter frame apparatus and method of the instant invention, two endless tracks guide individual active carriages in loops that are arranged opposite one another with the film passing between the loops. Synchronous secondaries are attached to these carriages to which are also attached tenter clips to grip the edges of the film. Elongated primaries are located opposed to each other on the film drawing or forward side of each loop and are adjacent the synchronous secondaries on the carriages to engage them electromagnetically. In a significant aspect of the invention, each primary includes a plurality of groups of coils with the group of coils in one primary sized to match the opposed groups of coils in the other primary and with each of the opposed groups of coils being electrically joined and defining a single control zone. Power is applied to these control zones to propel opposed pairs of carriages in symmetry through each control zone and from one control zone to the next throughout the tenter frame. The tracks in each loop can diverge as in a conventional tenter frame and simultaneously the carriages gripping the edges of the film can be separated in the MD and TD as the carriages travel along the track. The film is thereby simultaneously biaxially drawn or stretched.
The synchronous secondaries assure that the active carriages will synchronously engage, or lock onto, the electromagnetic wave developed by the primaries resulting from control instructions. Therefore, as long as the carriages are continuously fed to the forward sides of the loops, preferably in stacks being propelled at controlled speeds, and the opposed groups of coils in each control zone simultaneously receive alternating current developed from the same predetermined control instruction, which are simultaneously coordinated with adjacent control zones instructions, the carriages will remain in symmetry as they are continuously propelled along the forward sides of both loops.
The art mentioned above does not show or suggest the method just described.
The Habozit patent, and the French patent, which is related to it, only show an endless loop linear motor system where individually controlled field windings along opposed loops accelerate carriages containing film clips through a tenter oven. The induction secondaries are attached to each carriage to propel them throughout the loops. There is no teaching as to how to achieve symmetry of motion of opposed clips, nor is there any teaching of how to solve the unique problems of controlling a plurality of carriages having synchronous secondaries attached.
The Japanese patent publication 48-38779 only shows an apparatus to propel tenter clips in opposed loops for stretching film using a "short core-type linear motor". No means is provided to insure symmetry of motion of opposed pairs of clips.
The instant invention, by providing this controlled symmetry of motion, offers improvements over the tenter frame art.
This invention further involves the use of linear motors to propel carriages in endless loops from the exit of carriage collection sections, where they move abutted in stacks at a constant synchronous speed, to a second or greater speeds to space the carriages apart, after which such carriages are propelled back into the stacks prior to reaching the entrance of the collection sections. The speed at which the carriages move after contact with the stacks is determined by the speed of the abutted carriages in the carriage collection sections, against which the carriages newly arrived in the stacks are constantly pressing. The carriages move asynchronously after contact with the stacks and prior to reaching the entrance of the collection sections.
The movement of the active carriages into the stacks is controlled by hysteresis secondaries also attached to the carriages. These secondaries are engaged by electromagnetic waves independently developed by zones of linear motor primaries positioned adjacent the hysteresis secondaries. The electromagnetic waves may vary in speed to control acceleration and deceleration of the carriages.
Further, in another aspect of this invention, one or more passive or idler, clips may be placed between each of the active clips, in each loop of the tenter frame, to minimize film edge scalloping as shown, for example, in previously mentioned French patent 2,317,076. The carriages to which these passive clips are attached are unpowered during the film stretching operation. Such passive carriages are initially propelled by abutment with the active carriages having the synchronous secondaries attached thereto. As the active carriages separate to stretch the film, the passive clips are then propelled, on the forward sides of the tenter frame, by their engagement with the moving film.
After the film is released, these carriages are then returned, along with the active carriages, along the return sides of the loops and into the stacks of carriages, prior to movement back into the film processing section of the tenter frame. This is accomplished by attaching hysteresis secondaries to the passive carriages. These secondaries are adjacent the same primaries engaging the hysteresis secondaries on the active carriages. The electromagnetic waves developed by these primaries act to propel the carriages along the return sides and into the stacks before they reach those parts of the stacks being propelled at constant controlled speeds, in the carriage collection sections adjacent the entrance to the tenter frame. This use of hysteresis linear motors to propel the carriages on the return sides of the loops and into the stacks of carriages is an important feature of this invention.
Linear motors, of course, are known to the art and such motors can be used to propel carriages or tracked vehicles in an endless loop.
U.S. Pat. No. 3,893,466 to Starkey is one example of such a teaching, showing the use of a linear synchronous motor propulsion system for independently propelling tracked vehicles in a loop. The vehicles or incoming trains are selectively accelerated and then decelerated at an appropriate location to allow them to travel at a slow speed and at a selected spacing through a station.
U.S. Pat. No. 3,890,421 to Habozit is another example showing the use of a linear induction motor for controlling the speed of clamps mounted on carriages moving in endless loops for biaxially drawing plastic film. And Japanese patent application 48-38779 is still another example showing the use of a linear motor to propel tenter clips in endless loops to biaxially stretch thermoplastic synthetic resin films. There is no indication, however, as to how the carriages are controlled on the return sides of the loops.
It further is common practice, in systems involving carriages or vehicles traveling in an endless loop, to provide a loading station or startup section where the carriages are moved at low speeds for loading of passengers as in U.S. Pat. No. 3,803,466 or for other operations and are then accelerated in an operational section. In the startup section the carriages are closely spaced and frequently are clustered or grouped in a stack as shown in the above-mentioned patents, for example. After acceleration or completion of the operation, such as film stretching, the carriages are returned to the stack, or loading section, ready to start the operation again.
It is generally required that the carriages be under control at all times in their movement in and, through the loop. This is particularly true when the carriages or tracked vehicles, which are frequently moving at high speeds, are returned to the stack, otherwise damaging collisions may occur or machine operation may be affected.
Various techniques have evolved to solve this type of problem. One such solution is seen in U.S. Pat. No. 4,675,582 to Hommes and Keegan, owned by the assignee of the present invention. This patent, which is incorporated herein in its entirety by reference, discloses a linear synchronous motor control system which can be used to precisely propel synchronous secondaries attached to carriages at ever increasing speeds on the forward sides of a pair of opposed loops to stretch film, for example. This same system also can be used to decelerate the carriages under control on the return sides of such loops. In such a system where carriage speed and spacing is varying, there can never be more than one carriage in an electrically separate group of coil windings, or zone, of the primary at a time. This constraint requires many primary zones and their associated zone controls. Such a system effectively accomplishes the task of continuously propelling carriages throughout an endless loop, but at a significant cost in hardware and complexity, particularly on the return side where stacking occurs and where such precise synchronous control of the carriages may not be required.
In carriage or tracked vehicle propulsion systems the location and speed of the carriages at startup is frequently of prime importance. For example, the tracked vehicles in the Starkey patent mentioned above, appropriately travel at a selected spacing in the station, for loading purposes. This is true in other systems as well, including the system shown in the Hommes and Keegan patent just described, where control of the secondaries, in a constant velocity section, with their precise locations known prior to acceleration, is important in the operation of the system.
The instant invention, by assuring that the stacks of carriages move synchronously and abutted at controlled constant speeds in carriage collection sections, further assures that the carriages at startup will be in the proper position and that they will be moving at a proper controlled speed. In so doing, such invention provides an improved or alternate method for practicing the inventions of U.S. Pat. No. 3,803,466 and of U.S. Pat. No. 4,675,582, particularly on the return side, for example.
The location and speed of the carriages at startup is also of prime importance for drawing film in a tenter frame. For example, in stretching a web of film, it is important that the carriages to which the tenter clips are attached enter the tenter frame at a known spacing. This is true in other systems as well.
More specifically, in practicing the invention, the carriages must enter the tenter frame in synchronism with the electromagnetic wave in the first control zone. Prior to machine startup the carriages are pressed up against one another with the carriage bodies abutted and the lead carriage held stationary. This establishes the spacing at a known unvarying value at which the magnetic pole pitch of the carriage secondaries matches the electromagnetic wave pole pitch determined by the coils in the primary.
After this pre-startup orientation of carriages is established, and the remainder of the forward sides of the loops are empty of carriages, the tenter frame can be started up and the carriages will be propelled one after the other along the forward sides in synchronism with the electromagnetic waves and returned along the return sides. If the forward sides of the tenter frame are stopped in a controlled fashion, the relative positions of the carriages can be maintained and restarting does not require realigning of the carriages.
The instant invention by assuring that the stacks of carriages are moved synchronously and abutted in the carriage collection sections further assures that the carriages at startup will be in the proper position and that they will be moving at a proper controlled speed.
The tenter system of the invention also has means to alter the MD draw ratio while continuing to simultaneously biaxially draw the film. This permits threading-up film at low MD draw ratios and then gradually changing the MD simultaneous biaxial draw ratio to a higher level for continuous operation. The stack forming sections on the return sides of each loop can also gradually change the deceleration rate and stack length to accommodate the shift in numbers of carriages from the film processing sections to the stack forming sections as the MD draw ratios increase; which increase is also generally accompanied by a proportionate increase in peak carriage velocity that requires changes in deceleration rates. This unique feature to readily and rapidly change draw ratios also permits rapid, low cost optimization of film drawing ratios without having to shut down the line and fabricate and install new parts for new incrementally changed draw ratios. In commercial simultaneous biaxial film tenters, the simultaneous MD draw cannot be changed after start-up, so the simultaneous MD draw ratio at start-up and the simultaneous MD draw ratio for continuous operation have to be the same. For certain film polymers, however, there is the problem that film tearing occurs when threading-up at high draw ratios. This problem is overcome by the system of this invention. Furthermore, such invention offers precise predictable control of carriage motion with few moving parts and an open-loop (no feedback) control system, and without fixed-pitch mechanical screws and chains, or position and drive signal feedback systems. The instant simultaneous biaxial tenter frame can operate at much higher draw ratios and line speeds than previously possible.
Accordingly, this invention makes available to the art improved methods and apparatus for propelling carriages around loops in a tenter frame, and solves various problems heretofore confronting the art by providing an effective film drawing operation controlled by snychronous motors and by assuring that the carriages are moved back into stacks of carriages, in a controlled manner, using relatively inexpensive hysteresis motors. Such invention further assures that the carriages in those portions of the stacks in the carriage collection sections, are always abutted and that they too, are moving at a proper controlled constant speed, prior to entering the forward sides of the tenter frame.
Such invention represents a major advance in the art not only of linear motor systems, but of known systems for biaxially drawing plastic films in tenter frames.